(1) Field of the Invention
This invention relates to an austenitic stainless steel exhibiting the combination of excellent galling resistance in conventional wrought and annealed form, excellent stress corrosion resistance in chloride-containing environments, good resistance against intergranular corrosion, good high temperature oxidation resistance, good wear resistance, and a high work hardening rate. The alloy of this invention can be readily worked with conventional equipment into plate, sheet, strip, bar, rod and the like, and retains a substantially austenitic structure throughout a wide temperature range.
The steel of the invention is adapted to applications in which moving metal-to-metal contact, corrosive attack and/or high temperatures are encountered in combination. Although not so limited, the steel has particular utility for fabrication into roller chains, link belts on conveyors, valves subjected to elevated temperature, woven metal belts for continuous heat treating furnaces, fasteners, pins and bushings.
(2) Description of the Prior Art
Although galling and wear may occur under similar conditions, the types of deterioration involved are not similar. Galling may be defined as the development of a condition on a rubbing surface of one or both contacting metal parts wherein excessive friction between minute high spots on the surfaces results in localized welding of the metals at these spots. With continued surface movement this results in the formation of even more weld junctions which eventually sever in one of the base metal surfaces. The result is a build-up of metal on one surface, usually at the end of a deep surface groove. Galling is thus associated primarily with moving metal-to-metal contact and results in sudden catastropic failure by seizure of the metal parts.
On the other hand, wear is synonymous with abrasion and can result from metal-to-metal contact, or metal-to-non-metal contact, e.g. the abrasion of steel mining equipment by rocks and similar mineral deposits. Such wear is characterized by relatively uniform loss of metal from the surface, as contrasted to localized grooving with consequent metal build-up, as a result of rubbing a much harder metallic surface against a softer metallic surface. The distinction between galling and wear can perhaps best be illustrated by the fact that galling can be eliminated by mating or coupling a very hard metallic surface with a much softer metallic surface, whereas wear or abrasion in metal-to-metal contact would be increased by mating a very hard surface with a much softer one.
An article by Harry Tanczyn entitled "Stainless Steel Galling Characteristics Checked" in STEEL, Apr. 20, 1954 points out that stainless steel sections at a relatively high hardness level, or with a substantial difference in hardness, exhibit better resistance to galling than the combination of two soft members. This may be explained by the theory that the hardened sections deform elastically near the contact points under loading, while the softer pieces yield plastically for a significant distance beneath the contact points. During movement, the hardened surfaces apparently recover elastically with decrease in pressure, and this motion tends to sever any metallic welding. This article also indicates that good resistance to galling may be traceable to the combination of a suitable oxide surface film and a hard backing. Oxide films were found to influence the galling characteristics of metals, e.g. a film of Fe.sub.3 O.sub.4 increased the resistance of mild steel to galling, while a film of Fe.sub.2 O.sub.3 did not benefit resistance to galling.
Among the numerous prior art steels currently available, the austenitic AISI Type 304 is suited to a variety of uses involving welding and fabrication, but the galling and wear resistance of this steel are poor, and the metal is likely to fail when subjected to such conditions.
A precipitation-hardening stainless steel, sold under the registered trademark ARMCO 17-4 PH (about 15.4% chromium, about 4.0% nickel, about 4.0% copper, about 1.0% manganese, about 1.0% silicon, up to 0.07% carbon, 0.35% columbium, and remainder iron), while possessing high strength and hardness in the hardened condition, exhibits only fair galling and wear resistance.
U.S. Pat. No. 3,663,215, issued May 16, 1972 to H. Tanczyn, discloses a steel having improved wear resistance, which at the same time is weldable, workable, and/or machinable, and precipitation hardenable by heat treatment to great hardness. It has been found that this steel has good galling resistance. However, it contains large amounts of expensive alloying elements, and it is difficult to process with standard steel mill equipment. The broad composition ranges are about 10 to about 22% chromium, about 14 to about 25% nickel, about 5 to about 12% silicon, one or more of the elements molybdenum up to about 10%, tungsten up to about 8%, vanadium up to about 5%, columbium up to about 5% and titanium up to about 5%, these additional elements being in sum total of about 3 to about 12%. Carbon is present up to about 0.15% and nitrogen up to about 0.05%. In this alloy silicon is stated to form silicides of molybdenum, tungsten and the like, in finely dispersed form in the matrix of the precipitation-hardened steel. These silicides are of extreme hardness, thereby providing good wear resistance.
A prior art steel presently considered to have the best resistance to wear and galling is the straight chromium AISI Type 440C, containing about 16 to 18% chromium, about 1% maximum manganese, about 1% maximum silicon, about 0.75% maximum molybdenum, about 0.95 to 1.20% carbon, and remainder iron. This steel is hardenable by heat treatment but has poor corrosion resistance and poor formability. It is difficult to roll into plate, strip, sheet, bar or rod, and articles of ultimate use cannot be readily fabricated from plate, sheet, strip, bar or rod form.
U.S. Pat. No. 2,177,454, issued Oct. 24, 1939 to H. L. Frevert et al, discloses a valve steel for use in internal combustion engines, preferably containing from 0.10 to 1.0% carbon, over 10% and less than 20% chromium, 5 to 13% manganese plus nickel, the manganese being over 3% and less than 10.25% and nickel being over 1.75% and not over 3.5%, with the manganese content substantially exceeding the nickel content, 2.5 to 4.5% silicon or aluminum, the silicon being over 1.25%, and balance substantially iron.
U.S. Pat. No. 3,615,368, issued Oct. 26, 1971 to A. Baumel, discloses an austenitic steel alleged to have high resistance to stress crack corrosion and to solutions of nitric acid, containing up to 0.25% carbon, 1.5 to 10% silicon, up to 10% manganese, 13 to 30% chromium, 4 to 30% nickel, up to 10% molybdenum, 0.04 to 0.3% nitrogen, and balance iron and unavoidable impurities. The addition of nitrogen in an amount of 0.04 to 0.3%, and preferably from 0.08 to 0.2%, is stated to minimize formation of intermetallic deposits at the grain boundaries of the heat affected zone of a weld.
Reference is further made to U.S. Pat. No. 3,912,503 issued Oct. 14, 1975 to the present applicants and assigned to the assignee of the present application. The patent discloses an austenitic stainless steel having excellent galling resistance in conventional wrought form, good wear resistance, good corrosion resistance in chloride-containing environments, and a high work hardening rate. The steel of this U.S. patent consists essentially of from about 10 to about 25% (preferably about 12 to about 19%) chromium, about 3% to about 15% (preferably about 4 to about 12%) nickel, about 6 to about 16% (preferably about 7 to about 13%) manganese, about 2 to about 7% (preferably 3 to 5%) silicon, about 0.001 to about 0.25% (preferably about 0.01 to about 0.12%) carbon, about 0.001 to about 0.4% (preferably about 0.03 to about 0.3%) nitrogen, up to about 4% (preferably about 0.75% maximum) molybdenum, up to about 4% (preferably about 0.75% maximum) copper, a maximum of about 0.09% phosphorus, a maximum of 0.25% sulfur, a maximum of 0.50% selenium, and remainder essentially iron except for incidental impurities.
In our above-mentioned U.S. Pat. No. 3,912,503 the silicon addition is believed to modify the composition of the surface oxide film of the steel, making it more stable and adherent. Silicon is dissolved in an austenitic matrix. Moreover, the silicon addition exerts a substantial increase in the work hardening rate of the steel. Unlike the steel of the above mentioned U.S. Pat. No. 3,663,215, silicon does not form a silicide of molybdenum, tungsten, vanadium, columbium and/or titanium.